.\"	$NetBSD: __builtin_object_size.3,v 1.10 2012/07/19 06:44:12 wiz Exp $
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.\" by Christos Zoulas.
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.Dd July 18, 2012
.Dt __BUILTIN_OBJECT_SIZE 3
.Os
.Sh NAME
.Nm __builtin_object_size
.Nd return the size of the given object
.Sh SYNOPSIS
.Ft size_t
.Fn __builtin_object_size "void *ptr" "int type"
.Sh DESCRIPTION
The
.Fn __builtin_object_size
function is a
.Xr gcc 1
built-in function that returns the size of the
.Fa ptr
object if known at compile time and the object does not have any side
effects.
.Sh RETURN VALUES
If the size of the object is not known or it has side effects the
.Fn __builtin_object_size
function returns:
.Bl -tag -width (size_t)\-1 -offset indent
.It Dv (size_t)\-1
for
.Fa type
.Dv 0
and
.Dv 1 .
.It Dv (size_t)0
for
.Fa type
.Dv 2
and
.Dv 3 .
.El
.Pp
If the size of the object is known, then the
.Fn __builtin_object_size
function returns the maximum size of all the objects that the compiler
knows that they can be pointed to by
.Fa ptr
when
.Fa type
.Dv \*[Am] 2 == 0 ,
and the minimum size when
.Fa type
.Dv \*[Am] 2 != 0 .
.Sh SEE ALSO
.Xr gcc 1 ,
.Xr __builtin_return_address 3 ,
.Xr attribute 3 ,
.Xr ssp 3
.Sh HISTORY
The
.Fn __builtin_object_size
appeared in
.Tn GCC 4.1 .
.Sh CAVEATS
This is a non-standard, compiler-specific extension.
.Pp
Note that currently the object size calculation pass is only done at -O1
or above, meaning that this function always returns \-1 when the optimizer
is off.
.Pp
There are some discussions about always doing the object size pass, but
the issue is that without the optimization pass data sizes are not going
to be correct.
.Pp
For that reason currently code fortification (size-checked replacement
functions) is disabled when optimization is off.
